Referring to FIG. 1, an exemplary network environment is illustrated in which users can communicate data across distinct networks. In this example, the networks include packet networks, particularly first and second Voice over Internet Protocol (VoIP) networks 102, 104, a cellular communication network 106, and the publicly switched telephone network (PSTN) 108.
A subscriber device such as a personal digital assistant (PDA), a cellular phone, a VoIP phone, a smart phone, a plain old telephone service (POTS) phone, a laptop computer, etc. can access communication services from its respective network. For example, VoIP phones 110, 111, 130, 131 at the first and second Voice over Internet Protocol (VoIP) networks 102, 104 receive communication resources from their respective VoIP network by establishing a connection with a respective access point 112, 132. A POTS phone 114 can receive communication resources from the PSTN 108 by establishing a connection with a class 5 switch 116. A cellular phone 118 can receive communication resources from the cellular communication network 106 by establishing a connection with a base station 120. The cellular phone 118, POTS phone 114, and VoIP phones 110, 111, 130, 131 will all be referred to here as subscriber devices.
In one exemplary communication session between the subscriber device 131 at the first VoIP network 102 and the subscriber device 111 at the second VoIP network 104, the first subscriber device 131 encodes data such as voice, video or audio into packets according to a standard such as G.729, MPEG 4, or G.711 using a codec (coder-decoder) and transmits the encoded data to the second VoIP network 104. The second subscriber device 111 receives and decodes the packets into data by using a codec implementing a compatible standard. Here, the terminology encoding and decoding data will be considered similar to compressing and decompressing data.
Packet loss can occur while the packets are being transmitted over the packet networks due to factors such as signal degradation over the network medium, oversaturated network links, corrupted packets rejected in-transit, faulty networking hardware, maligned system drivers or network applications, or normal routing routines. Further, the service area defined by an access point of the VoIP network can be limited by poor radio link quality conditions, such as, for example, interference due to microwave ovens or radio technologies in the unlicensed 2.4 GHz band. Packet loss can result in significant degradation in the quality of a communication session.
Packet loss does not only occur during communication sessions in which the transmitting and receiving subscriber devices are both at distinct VoIP networks. For example, when a transmitting subscriber device transmits packets over a packet network to a receiving subscriber device at the cellular network or the PSTN, the packets will be decoded according to the compatible standard and re-encoded according to Pulse Code Modulation (PCM) at a media gateway, and transmitted over the PSTN. Here, any packet loss will also affect the quality of the PCM re-encoded data.
Packet loss concealment (PLC) can be performed to mask the effects of packet loss. However, PLC at the receiving subscribing device is not sufficient for recovering all of the packet loss. Other approaches such as transmitting extra information can improve the quality of speech reconstructed at the receiving end, but also have the drawback of higher bandwidth consumption.